1. Field of the Invention
The present invention generally relates to the art of optoelectronic semiconductor devices, and more particularly to a floating base lateral bipolar phototransistor which incorporates field effect gate voltage control to increase operating frequency and gain.
2. Description of the Related Art
High gain optical detectors or imaging arrays are used in sensors, video, imaging, and thermal/optical mapping in a variety of aerospace, commercial, and other applications. High speed photodetectors with electrically controlled gain and responsivity are used in optical communication systems, local area networks and various optical sensing and imaging applications. Photodetectors which are capable of high speed and high gain operation and which detect optical radiation from the deep ultraviolet (200 nm) to visible (800 nm) spectral regions are especially desirable.
Known optoelectronic semiconductor devices include photodiodes, bipolar phototransistors, and field effect (FET) phototransistors. These devices operate on the principle that irradiation of a semiconductor material with light causes liberation of electron-hole pairs in the material. This causes increased current flow through the devices in a desired manner according to the design criteria.
Photodiodes are limited in that they do not produce amplification. Field effect phototransistors suffer from the drawback of low frequency response in bulk silicon substrates.
Conventional bipolar phototransistors formed in bulk silicon substrates have low frequency response, low common emitter gain (.beta.), and cannot effectively absorb deep ultraviolet radiation in the region of high gain (away from the Si/SiO.sub.2 interface). These limitations are primarily due to the device parasitics (capacitance between base-emitter and base-collector junctions, and base series resistance) and the lack of a base region directly exposed to the optical radiation.
The parasitic capacitances in bipolar devices can be reduced by forming the active regions in a silicon layer on an insulator substrate. A number of processes are available for forming silicon-on-insulator (SOI) substrates, including Separation by Implanted Oxygen (SIMOX), wafer bonding, and Zone-Melting-Recrystallization (ZMR). Crystallinity of this material can be improved by Double Solid Epitaxy (DSPE) or a new solid state epitaxy and regrowth (SPEAR) process disclosed by the present inventor in U.S. Pat. No. 4,509,990.
Bipolar devices in general and Metal-Oxide-Semiconductor (MOS) devices are normally considered to be separate and distinct, with each having its own advantages and disadvantages.
A major advantage of bipolar devices formed in silicon or GaAs is their ability to operate at higher gain than MOS devices. However, conventional bulk bipolar devices typically have fixed low gain, which is a disadvantage in many circuits since additional components may be needed to increase the overall gain of the circuit. In addition, bipolar devices require a more complex fabrication process than MOS devices, and occupy considerably more space than MOS devices on an integrated circuit chip.
Although MOS devices are limited in current handling capacity relative to bipolar devices, they have the major advantages of simplicity of fabrication and high packing density. Although bipolar and MOS devices have been formed on single integrated circuit chips to produce "BIMOS" circuits, such known devices have been difficult and expensive to manufacture reliably due to the necessity of using both bipolar and MOS masking and doping steps during the fabrication process. Optoelectronic semiconductor devices which combine the advantages of bipolar and MOS technologies while avoiding the drawbacks thereof have been unknown in the art.